1. Field of the Invention
The present invention is directed to optical fibers for transmitting optical signals and, more particularly, to improved multimode optical fibers that possess unique refractive index profiles. Optical fibers according to the invention possess high bandwidth and high numerical aperture at multiple wavelengths while favorably accounting for material dispersion effects.
2. Description of the Related Art
There are major needs for high bandwidth, high numerical aperture optical fibers for applications in short and medium distance fiber optic communications network systems. Bandwidth is the amount of information that can be transmitted in a specified time interval and is among the most important characteristic features of an optical fiber link. The current ever-growing demand for high bandwidth is for voice, video, and data transmission in short and medium distance applications such as local area networks (LANs) and metropolitan area networks (MANs). In particular, current high data rate network forms, such as asynchronous transfer mode (ATM) and Ethernet, operate at several hundred megahertz (MHz) with bandwidths of 1000 MHZ (1 gigahertz (GHz)) and greater already planned for implementation.
Step index fibers according to the prior art are characterized by a single core region having a uniform refractive index, surrounded by a cladding layer having a lower refractive index. Graded index fibers according to the prior art are characterized by a single core region having a continuously varying refractive index from a higher value at the center of the fiber to a lower value at the core-cladding boundary. The graded core region is also surrounded by a cladding layer.
High numerical aperture multimode step index (SI) glass optical fiber can operate at data rates of 10-100 MHZ, but these lower rates are already approaching the physical bandwidth limits of the fiber. Standard single mode glass optical fiber can possess much higher bandwidth, as high as 5-10 GHz, but the prohibitive costs of splicing and connecting the several micron diameter fiber cores work against their wide spread use in LAN and MAN applications where multiple connections are required. Low numerical aperture, difficult to connect, multimode graded index (GI) glass optical fiber can also provide a higher bandwidth of approximately 1 GHz, but only at selected operating wavelengths. A multimode graded index glass optical fiber exhibits different bandwidth performance at different wavelengths due to the effect of material dispersion.
It is well established for optical fibers that fiber bandwidth performance is determined in large measure by the specific characteristics of the refractive index profile. In the presence of material dispersion, these index profile characteristics are different under different operating wavelengths. For example, it is desirable to operate silica based fibers at approximately 850, 1300, and 1550 nanometers (nm). Attempts to optimize bandwidth performance at multiple wavelengths have had limited success because of the complicated material compositions and fabrication conditions needed in forming graded refractive index profiles. It is well known that the precise control of the graded refractive index profiles required, for example, in presently available graded index glass optical fibers is a difficult and challenging task. The above problems and shortcomings of graded index silica based, i.e., glass, optical fibers apply equally to graded index polymer optical fibers.